This invention relates to a power transmission device for an agricultural tractor.
In an agricultural tractor, the engine is usually mounted on its front portion of the body and a transmission case is usually mounted on the rear portion. FIG. 1 is a side view showing such a conventional agricultural tractor, in which 01 denotes a rear wheel and 02 denotes a front wheel. The front part of the tractor body is provided with an engine 03 and the rear part thereof is provided with a transmission case 04. The main shaft of the engine 03 is connected to a propeller shaft 06 through a reduction gear 05. The propeller shaft 06 extends backwards along the bottom of the tractor body and is connected to an input shaft 07 of the transmission case 04.
A main shifting device 08 and a secondary shifting device 09 are provided within the transmission case 04 in the front and back thereof, respectively. Engine power is transmitted by the input shaft 07 to a differential shaft 010 through a differential apparatus with an accompanying increase or decrease in rotational speed and further to a rear axle 011 through a final reduction gear. The power, which passes through the main and secondary shifting devices 08 and 09, is also transmitted to the front wheel 02 through a front-wheel drive propeller shaft 012. Furthermore, the engine power, which is input to the input shaft 07, is properly changed in speed and then transmitted to a PTO shaft 013. The PTO shaft 013 transmits power to a piece of agricultural machinery such as a rotary machine which is connected to a rear part of the tractor.
The agricultural machinery is connected to the tractor through a lifting arm 015 which is pivotably supported by a hydraulic cylinder 014 which is mounted on the upper portion of the transmission case 04 and is vertically swung in association with the movement of a piston of the hydraulic cylinder and a lower arm 016 which is pivotably supported by the side of the transmission case 04 and is lifted up and down by the hydraulic cylinder 014.
In such a conventional tractor, since the differential shaft 010 and rear axle 011 are disposed to the rear of the secondary shifting device 09, the length of the transmission case 04 in the longitudinal direction is increased by that amount. Furthermore, since the lower arm 016 must be connected to the side wall of the transmission case 04 to the rear position of the differential shaft 010 so that the lower arm 016 will not interfere, when it is swung vertically, with the differential shaft 010 which extends from the side wall of the transmission case 04, the agricultural machinery ends up being separated too far from the body of the tractor. Accordingly, when performing plowing, for example, the area which is left unplowed is comparatively large, and it is necessary to make another pass through a field to plow those areas which were missed.
Japanese Utility Model Laid-Open Publication No. 47-27295 discloses a transmission device for a tractor in which a differential apparatus is disposed between a main shifting device and a secondary shifting device. In this transmission device, three shafts are disposed in three stages, one in an upper stage, the second in an intermediate stage, and the third in a lower stage. Both end portions of an upper shaft and a lower shaft, which extend in the longitudinal direction through the transmission case, are provided with a shifting gear. Accordingly, the height of the transmission case becomes large over its entire length from its front to its rear. As a result, the seat, which is disposed above the transmission case, must also be high due to the height of the transmission case. Furthermore, in this device, a PTO shaft, which must be disposed at a predetermined height in relation with a piece of agricultural machinery, is disposed in a lower part of the transmission case. Accordingly, the transmission case must be placed in a high position due to the height of this PTO shaft. As a result, the height of the seat is still further increased.
Furthermore, in the tractor, a brake apparatus is interposed between a differential shaft and a rear axle on both sides of the tractor body, and the tractor body is able to turn by separately actuating these brake apparatuses. This brake apparatus, as disclosed, for example, in Japanese Patent Laid-Open Publication No. 59-160623 and Japanese Utility Model Laid-Open Publication No. 53-50984, is provided on the outer wall of the transmission case, and foreign matter such as mud, and rain drops can easily enter into the brake apparatus.